It looks like if you modify the "My Appointments" user filter below...

To sync all non-recurring appointments, as shown below....

The user still only sees the appointments created by themselves in Dynamics 365.
The only Dynamics 365 appointments that show up in the user's Outlook calendar are the ones created by themselves in Dynamics 365.
If a second user creates an appointment in Dynamics 365, it doesn't show up in the first user's Outlook calendar.
Is this a limitation of the server-side sync feature?
Is there a way to sync more appointments than just the ones created by the user?